It is known to form a flexible corrugated tube by helically curling a strip of very thin metal which has longitudinal corrugations in the form of curved ridges and hollows. Such a tube and a method of making it is disclosed in Siegwart U.S. Pat. No. 4,141,385, granted Feb. 27, 1979. Another patent directed to similar subject matter is Westerbarkey 3,815,639, granted June 11, 1974.
The most desirable corrugated strip material for forming flexible corrugated tube is aluminum which is very thin (about 0.08 mm, or 0.003 in.), so the margins of the strip are very easily distorted during handling. This may result in a finished duct that looks defective but is not, or in a finished duct that actually is defective.